Financing Arrangements (Details) - USD ($) $ in Millions		1 Months Ended	12 Months Ended
	Mar. 28, 2013	Apr. 30, 2015	May. 31, 2015	May. 31, 2014	May. 31, 2013	Mar. 24, 2015	Apr. 12, 2011
Financing Arrangements
Total debt			$ 154.0	$ 634.0
Current maturities of debt			(69.0)	(69.7)
Long-term debt			85.0	564.3
Aggregate par value of debt maturing in next five fiscal years
2016			70.8
2017			10.0
2018			0.0
2019			25.0
2020			50.0
Outstanding letter of credit			15.4
Short-term borrowing activity
Net proceeds from offering of notes					$ 181.8
Loss on Repurchase			44.9		0.3
Interest expense
Amortization of discount			2.7	5.3	9.6
Loss on extinguishment of debt			$ (44.9)		(0.3)
Credit Facility Amendment
Interest expense
Maximum borrowing capacity under the credit agreement before amendment							$ 475.0
Extended maturity period			2 years
Maximum borrowing capacity						$ 500.0
Revolving Credit Facility expiring March 24, 2020 with interest payable monthly
Financing Arrangements
Total debt			$ 50.0	130.0
Revolving Credit Facility expiring March 24, 2020 with interest payable monthly | Eurodollar rate | Minimum
Aggregate par value of debt maturing in next five fiscal years
Basis spread on variable rate under the credit agreement after amendment (as a percent)			1.00%
Revolving Credit Facility expiring March 24, 2020 with interest payable monthly | Eurodollar rate | Maximum
Aggregate par value of debt maturing in next five fiscal years
Basis spread on variable rate under the credit agreement after amendment (as a percent)			2.00%
Revolving Credit Facility expiring March 24, 2020 with interest payable monthly | Base rate | Minimum
Aggregate par value of debt maturing in next five fiscal years
Basis spread on variable rate under the credit agreement after amendment (as a percent)			0.00%
Revolving Credit Facility expiring March 24, 2020 with interest payable monthly | Base rate | Maximum
Aggregate par value of debt maturing in next five fiscal years
Basis spread on variable rate under the credit agreement after amendment (as a percent)			1.00%
Industrial revenue bond (secured by property, plant and equipment) due August 1, 2018
Financing Arrangements
Total debt			$ 25.0	25.0
Long-term debt:
Net carrying amount			0.0	29.9
Secured credit facility (secured by aircraft and related engines and components) due April 23, 2015 with floating interest rate, payable monthly
Aggregate par value of debt maturing in next five fiscal years
Amount outstanding				$ 2.7
Interest expense
Maximum borrowing capacity under the credit agreement before amendment	$ 65.0
Maximum borrowing capacity	$ 40.0
Secured credit facility (secured by aircraft and related engines and components) due April 23, 2015 with floating interest rate, payable monthly | LIBOR
Aggregate par value of debt maturing in next five fiscal years
Description of variable rate basis	LIBOR			LIBOR
Basis spread on variable rate (as a percent)	1.75%			1.75%
Basis spread on variable rate under the credit agreement before amendment (as a percent)	3.25%
Note payable due March 9, 2017 with floating interest rate, payable semi-annually on June 1 and December 1
Financing Arrangements
Total debt			$ 20.0	$ 30.0
Notes payable due January 15, 2022 with interest at 7.25% payable semi-annually on January 15 and July 15
Financing Arrangements
Interest rate (as a percent)		7.25%	7.25%
Total debt				332.6
Short-term borrowing activity
Loss on Repurchase		$ (44.9)
Interest expense
Redemption value of senior notes		325.0
Redemption value including make-whole premium		370.6
Loss on extinguishment of debt		44.9
Make-whole premium of redeemed notes		45.6
Unamortized deferred financing cost		6.2
Unamortized deferred cost offset		$ 6.9
Convertible notes payable
Interest expense
Coupon interest			$ 1.5	3.1	3.7
Amortization of deferred financing fees			0.1	0.3	0.5
Amortization of discount			2.4	5.1	9.3
Interest expense related to convertible notes			$ 4.0	8.5	13.5
Convertible notes payable due February 1, 2015 with interest at 1.75% payable semi-annually on February 1 and August 1
Short-term borrowing activity
Effective interest rate (as a Percent)			1.75%
Long-term debt:
Net carrying amount			$ 0.0	29.8
Convertible notes payable due March 1, 2016 with interest at 2.25% payable semi-annually on March 1 and September 1
Financing Arrangements
Interest rate (as a percent)			2.25%
Total debt			$ 48.0	45.7
Mortgage loan (secured by Wood Dale, Illinois facility) due August 1, 2015 with interest at 5.01%
Long-term debt:
Net carrying amount			11.0	11.0
Other
Financing Arrangements
Total debt			11.0	70.7
7.25% bonds, 1.75% and 2.25% convertible notes
Aggregate par value of debt maturing in next five fiscal years
Carrying value of long-term debt			48.0
Long-term debt:
Principal amount			48.0
7.25% bonds, 1.75% and 2.25% convertible notes | Significant other observable inputs (Level 2)
Aggregate par value of debt maturing in next five fiscal years
Estimated fair value of long-term debt			51.5
Remaining variable rate and fixed rate debt | Significant other unobservable inputs (Level 3)
Aggregate par value of debt maturing in next five fiscal years
Estimated fair value of long-term debt			106.0
Credit Agreement and its Predecessor
Short-term borrowing activity
Maximum amount borrowed			215.0	190.0	395.0
Average daily borrowings			$ 140.7	$ 135.8	$ 312.3
Average interest rate during the year (as a percent)			1.69%	1.77%	2.03%
Revolving credit facility | Credit Facility Amendment
Interest expense
Increase in borrowing capacity			$ 250.0
Total borrowing capacity			$ 750.0